Ok, as of 20 minutes ago, I'm experiencing the exact same problem. As I can't even manually lock the rear door on the driver's side, I did spray with wd-40 as I had it with me.
Does not seem to have worked and I noticed in the post above it said not to use that. If someone could tell me specifically what kind of grease to go get and if i need to take any of the door apart to apply it, I would greatly appreciate it.